1. Field of the Invention
The present invention generally relates to an automobile underbody structure and, more particularly, to a rear underbody structure where a rear bumper is installed.
2. Description of the Prior Art
A compact station wagon of a type wherein three seat assemblies are installed inside the compartment, front, intermediate and rear seat assemblies, is now gaining a popularity in the market. In this three-seat model, opposite side frames or sills extending longitudinally of the automobile body structure and laterally spaced have their rear portions bent so as to extend at a level offset upwardly relative to front portions thereof, and the rear seat assembly is installed atop the upwardly offset rear portions of the side frames.
With this three-seat station wagon, it has been found that the presence of respective slantwise portions, or kick-up regions, of the side frames intermediate between the front and rear portions tends to facilitate upward buckling of the upwardly offset rear portions of the side frames relative to the front portions of the side frames, rather than allowing them to buckle downwards, when an impact of a value greater than a predetermined value is applied from the rear to the upwardly offset rear portions of the side frames as a result of a rear-end collision.
The impact resulting from the rear-end collision is generally applied axially to the side frames as discussed in the Japanese Laid-open Utility Model Publication No. 61-124441 published in 1986. However, in a compact station wagon, or any other automobile such as a van, particularly a front-engine rear-drive model, of the type wherein the offset amount of each rear portion of the side frames offset upwardly relative to the associated front portion of the side frame is relatively great for accommodating the associated rear wheel suspension system and, also, a rear bumper is required to be fitted at a relatively low level for the purpose of the automobile body style, the necessity often arises that the rear bumper be fitted offset upwardly relative to bumper brackets through which the impact is transmitted to the respective side frames. Accordingly, in those types of automobiles, means are necessitated to avoid any possible reduction in height between the floor panel and the roof which would otherwise result from the upward buckling of the rear portions of the side frames as discussed above.
To minimize the upward buckling of the rear portions of the side frames in the event of a rear-end collision, each of the side frames has hitherto been reinforced by providing the respective side frame with a sufficient wall thickness and/or by fitting an extra skeletal member to the respective side frame. The use of the side frames having increased wall thicknesses and/or reinforced by the extra skeletal members results in the increased weight of the automobile and also increased manufacturing cost thereof.
Apart from the problem associated with the side frames discussed above, the Japanese Laid-open Utility Model Publication No. 56-143949, published Oct. 30, 1981, discloses a rear bumper for use in an automobile of the type having a back door hinged to a rear edge portion of the roof for selective movement between open and closed positions, for example, that of station wagon, hatchback or van. According to this publication, a rearmost cross member extending between rear frames or sills at the rear of the automobile is fitted with a sealing member operable to seal a gap between the back door and the automobile rear underbody structure when the back door is moved to the closed position. The invention disclosed in this publication is characterized in that the sealing member is an integral part of the rear bumper. The rear bumper having the sealing member is fitted to rear frame extension members, which are separate from the spaced apart side frames or sills forming parts of the automobile underbody structure and which are therefore rigidly secured to respective rear ends of the side frames or sills.
It has, however, been found that the rear underbody structure disclosed in the Japanese publication, referred to above, in combination with the respective bumper structure has some problems which will now be discussed.
In the case of a station wagon, hatchback, van or any other type of automobile having a back door for the rear access opening leading into the rear compartment or luggage room, a rear-end collision often brings about detrimental damage to the automobile. While the primary function of the rear bumper is to minimize the damage to the automobile rear body structure, the integration of the sealing member, which also functions as a weatherstrip, with the rear bumper such as disclosed in the second mentioned Japanese publication, is apt to bring about a problem in that application of even a slight impact force as a result of a rear-end collision may destroy the sealability of the rear access opening.
The integration of the sealing member with the rear bumper may reduce not only the manufacturing cost which would be incurred in making the sealing member and the rear bumper separately, but also the number of steps necessary to install the sealing member and the rear bumper to the automobile rear body structure. However, any possible deformation of the rear bumper is accompanied directly by corresponding deformation of the sealing member.
The Japanese Laid-open Utility Model Publication No. 60-6360, published Feb. 28, 1985, discloses a rear bumper assembly for use in an automobile comprising a bumper surfacing of generally U-shaped cross-section, a generally elongated bumper reinforcement or stay made of fiber reinforced synthetic resin and an impact absorbing member of generally honeycomb-shaped structure sandwiched between the bumper surfacing and the bumper stay. This rear bumper assembly is fitted to spaced apart rear frame extensions of respective side frames, which form parts of the automobile underbody structure and which extend outwardly and rearwardly from the rear of the automobile body structure, and are positioned offset upwardly relative to the rear frame extensions.
The upwardly offset disposition of the rear bumper assembly relative to the rear frame extensions disclosed in the last-mentioned publication appears to have a problem in that the rear bumper assembly is apt to be displaced upwardly in the event of a rear-end collision. This is particularly true where the automobile rear underbody structure is of a design wherein a rear portion of each of the side frames is bent so as to extend at a level offset upwardly relative to the opposite front portion of the respective side frame for the purpose of accommodating a respective rear wheel suspension system and the associated rear wheel.